MINOR: init: add the pre-check callback

This adds a call to function <fct> to the list of functions to be called at
the step just before the configuration validity checks. This is useful when you
need to create things like it would have been done during the configuration
parsing and where the initialization should continue in the configuration
check.
It could be used for example to generate a proxy with multiple servers using
the configuration parser itself. At this step the trash buffers are allocated.
Threads are not yet started so no protection is required. The function is
expected to return non-zero on success, or zero on failure. A failure will make
the process emit a succinct error message and immediately exit.

doc/internals/api/initcalls.txt

@@ -114,6 +114,19 @@ make sure to respect this ordering when adding new ones.
   exit. See also hap_register_per_thread_alloc() for functions called before
   these ones.
 
+- void hap_register_pre_check(int (*fct)())
+
+  This adds a call to function <fct> to the list of functions to be called at
+  the step just before the configuration validity checks. This is useful when you
+  need to create things like it would have been done during the configuration
+  parsing and where the initialization should continue in the configuration
+  check.
+  It could be used for example to generate a proxy with multiple servers using
+  the configuration parser itself. At this step the trash buffers are allocated.
+  Threads are not yet started so no protection is required. The function is
+  expected to return non-zero on success, or zero on failure. A failure will make
+  the process emit a succinct error message and immediately exit.
+
 - void hap_register_post_check(int (*fct)())
 
   This adds a call to function <fct> to the list of functions to be called at
@@ -317,6 +330,10 @@ alphanumerically ordered:
   create_pool_callback() with these arguments at stage STG_POOL. Do not use it
   directly, use either DECLARE_POOL() or DECLARE_STATIC_POOL() instead.
 
+- REGISTER_PRE_CHECK(fct)
+
+  Registers a call to register_pre_check(fct) at stage STG_REGISTER.
+
 - REGISTER_POST_CHECK(fct)
 
   Registers a call to register_post_check(fct) at stage STG_REGISTER.

include/haproxy/init-t.h

@@ -6,6 +6,11 @@
 struct proxy;
 struct server;
 
+struct pre_check_fct {
+	struct list list;
+	int (*fct)();
+};
+
 struct post_check_fct {
 	struct list list;
 	int (*fct)();

include/haproxy/init.h

@@ -7,6 +7,7 @@
 struct proxy;
 struct server;
 
+extern struct list pre_check_list;
 extern struct list post_check_list;
 extern struct list post_proxy_check_list;
 extern struct list post_server_check_list;
@@ -18,6 +19,7 @@ extern struct list server_deinit_list;
 extern struct list per_thread_free_list;
 extern struct list per_thread_deinit_list;
 
+void hap_register_pre_check(int (*fct)());
 void hap_register_post_check(int (*fct)());
 void hap_register_post_proxy_check(int (*fct)(struct proxy *));
 void hap_register_post_server_check(int (*fct)(struct server *));
@@ -30,6 +32,10 @@ void hap_register_per_thread_init(int (*fct)());
 void hap_register_per_thread_deinit(void (*fct)());
 void hap_register_per_thread_free(void (*fct)());
 
+/* simplified way to declare a pre-check callback in a file */
+#define REGISTER_PRE_CHECK(fct) \
+	INITCALL1(STG_REGISTER, hap_register_pre_check, (fct))
+
 /* simplified way to declare a post-check callback in a file */
 #define REGISTER_POST_CHECK(fct) \
 	INITCALL1(STG_REGISTER, hap_register_post_check, (fct))

src/haproxy.c

@@ -1891,6 +1891,7 @@ static void init(int argc, char **argv)
 	struct wordlist *wl;
 	struct proxy *px;
 	struct post_check_fct *pcf;
+	struct pre_check_fct *prcf;
 	int ideal_maxconn;
 
 	if (!init_trash_buffers(1)) {
@@ -2107,6 +2108,8 @@ static void init(int argc, char **argv)
 	/* destroy unreferenced defaults proxies  */
 	proxy_destroy_all_unref_defaults();
 
+	list_for_each_entry(prcf, &pre_check_list, list)
+		err_code |= prcf->fct();
 
 	err_code |= check_config_validity();
 	for (px = proxies_list; px; px = px->next) {

src/init.c

@@ -4,6 +4,17 @@
 #include <haproxy/init.h>
 #include <haproxy/list.h>
 
+/* These functions are called just before a config validity check, which mean
+ * they are suited to use them in case we need to generate part of the
+ * configuration. It could be used for example to generate a proxy with
+ * multiple servers using the configuration parser itself. At this step the
+ * trash buffers are allocated.
+ * The functions must return 0 on success, or a combination
+ * of ERR_* flags (ERR_WARN, ERR_ABORT, ERR_FATAL, ...). The 2 latter cause
+ * and immediate exit, so the function must have emitted any useful error.
+ */
+struct list pre_check_list = LIST_HEAD_INIT(pre_check_list);
+
 /* These functions are called just after the point where the program exits
  * after a config validity check, so they are generally suited for resource
  * allocation and slow initializations that should be skipped during basic
@@ -73,6 +84,20 @@ struct list per_thread_free_list = LIST_HEAD_INIT(per_thread_free_list);
  * valgrind mostly happy. */
 struct list per_thread_deinit_list = LIST_HEAD_INIT(per_thread_deinit_list);
 
+/* used to register some initialization functions to call before the checks. */
+void hap_register_pre_check(int (*fct)())
+{
+	struct pre_check_fct *b;
+
+	b = calloc(1, sizeof(*b));
+	if (!b) {
+		fprintf(stderr, "out of memory\n");
+		exit(1);
+	}
+	b->fct = fct;
+	LIST_APPEND(&pre_check_list, &b->list);
+}
+
 /* used to register some initialization functions to call after the checks. */
 void hap_register_post_check(int (*fct)())
 {